The present invention relates generally to radiographic imaging devices, and more specifically to a system and method for mitigating the distortion effects of larger spot sizes thereby allowing the image quality of the radiographic imaging device to be improved. The present invention is particularly applicable to X-ray imaging technologies operating in the megavolt (MV) range, such as imaging technologies utilized in radiation therapy.
The quality of X-ray images produced by a radiographic imaging device is determined by a variety of factors. One factor is the size of the focal spot of the source that emanates the X-ray radiation. X-ray tubes for diagnostic applications operating somewhere between 80 kV and 120 kV of incident electron energies have made substantial progress over the years, attaining spot-sizes of much less than 1 mm, which in turn leads to good image quality in diagnostic X-ray imaging.
With the introduction of imaging technologies in radiation therapy, X-ray imaging in the megavolt range, with photon energies between 6 MV and 24 MV, is of significant interest. Linear accelerators for radiation therapy, however, were initially not designed as imaging devices, and hence no substantial engineering efforts have been expended to reduce the spot size of those accelerators to spot sizes provided by conventional X-ray tubes. When a linear accelerator is used as the X-ray source for imaging, it is more difficult to control the electrons to a very small spot size. In radiation therapy, where high dose rates are needed, larger spot sizes may be more desirable from an engineering point of view, since spot sizes which are too small might overheat the target locally.
For these reasons, spot sizes in megavolt (MV) imaging are considerably larger than spot sizes in kilovolt (kV) diagnostic imaging and have diameters significantly larger than 1 mm. The effect of these larger spot sizes is an inherent blurring of the images generated by the radiographic imaging device. Since reducing the spot size of medical linear accelerators below a certain size may not be practical or desirable, it would be desirable to develop apparatus and methods for reducing or mitigating the effects of larger spot sizes rather than reducing the spot size itself.